This invention relates to testing antennas, and more particularly, this invention relates to testing antenna gain.
Antennas are used for a wide variety of communications, radar and other applications, both in a transmit and in a receive mode. Antennas may take many shapes and forms, from simple whips to complex reflector schemes, to phased arrays, and may be used on the earth""s surface, in the air or in space. No matter what type of application, antennas are principally characterized by a broad set of gain or gain-related parameters. These include primary gain (in the intended direction, and relative to some reference standard such as isotropic), gain patterns over all angles from the intended direction, and frequency response (gain as a function of frequency). A typical requirement is to achieve as much gain as is possible in the intended direction, while minimizing gain in all other directions. Peaks of gain in unintended directions, called side lobes, usually must be minimized by design. For some applications, a desired specific frequency response must be attained, often seeking as wide a bandwidth as possible. For these reasons, antenna testing must be conducted during antenna development, adjustment or maintenance to measure gain and frequency response (including phase and amplitude response).
The amount of antenna gain is important due to the fact that antenna gain tests can be effected by extraneous reflections off walls and by other signals. Antenna gain testing and any related pattern testing typically occurs in an anechoic chamber, where many parameters can be measured, such as the antenna gain and the frequency/phase response. Typically, the anechoic chamber is a building that is designed and manufactured to have few echoes, such as those produced by signal reflections from natural and man-made objects. The chamber surface is covered with electromagnetically absorbing cones, which absorb any reflective signals. The anechoic chamber is also designed so that the area is free of extraneous signals, such as citizen band radio signals and other interfering or jamming signals. Naturally, these anechoic chambers are very expensive.
One conventional approach used for testing for antenna gain and pattern is to place an antenna-under-test in the anechoic chamber, together with a test antenna, and transmit a radio frequency or microwave signal from one antenna to the other antenna (in either direction). After the signal is received within the antenna, a receiver measures antenna gain through appropriate means known to those skilled in the art, such as possibly using a spectrum analyzer. In some instances, the frequency versus phase response is determined. However, reflections off the wall of the test signal sometimes cause extraneous results. Thus, unless alternatives are found for the very stringent design requirements necessary for operating anechoic chambers for testing antenna gain and pattern, it is mandatory that large expenditures of personnel time, money and other resources be placed into the design, testing, manufacture and operation of these sophisticated anechoic chambers.
It is, therefore, an object of the present invention to provide a system and method of testing for antenna gain and pattern either without using an anechoic chamber at all or using one having less stringent design requirements.
It is still another object of the present invention to provide a system and method of testing for antenna gain that is not prone to deviant measurements due to extraneous signals in the nearby environment.
In most measurement cases due to reciprocity, it is possible and practical to either transmit the test signals from a calibrated test transmit antenna to the antenna under test, used in a receive mode, or to transmit test signals from the antenna under test to a calibrated test antenna used in a receive mode. All subsequent explanations of this invention claim that either approach may be used as an implementation of this invention.
In the prior art, measurements were generally made using simple test signals, either sine waves of essentially zero bandwidth, or more rarely, truly random noise signals. The technique of this invention is to generate a spread spectrum (SS) pseudo-noise (PN) test signal wherein a test receiver can take advantage of the determinism of the PN signal to recover useful information about the antenna under test, but while rejecting undesired signals such as reflections or interference.
To measure antenna gain or angular patterns at a single frequency F1, a PN modulated signal is generated with a center frequency of F1, and with a bandwidth substantially within the bandwidth of the antenna under test. The characteristics of this PN modulation are such that the received signal may be collapsed in bandwidth to essentially zero bandwidth at F1, or at a down-converted frequency corresponding to F1, by carefully setting the locally-generated SS replica at a delay time xcfx84 corresponding to the delay from transmit to receive, including all path delays. The energy present at F1, or at a down-converted frequency corresponding to F1, is measured typically by a receiver signal strength indication or by spectrum analyzer, and is converted by calculation to an estimate of antenna gain, with all the other usual test factors considered. In doing so, reflections and interfering signals are substantially rejected because they are not correlated with the test signal.